Project Summary Multidrug resistance (MDR) in pathogenic bacteria poses a serious public-health problem. Previously, we have discovered cefoxitin can render methicillin-resistant S. aureus (MRSA) strains sensitive again to oxacillin. We thus hypothesize that small molecules may augment the bactericidal activity of ?-lactams against Gram+ pathogens. After screening a ~60,000 small- molecule library, we obtained candidate compounds with activity against MRSA with sub-MIC oxacillin. While one of the compound DNAC-2 has broad-spectrum activity and low level of toxicity, its core structure shares similarity to 5-hydroxy-quinoline, known to have anti-bacterial activity. To develop novel derivatives, we synthesized a series of unique analogs one of which is called DNAC-23a. In combination with oxacillin, DNAC-23a led to a 64-fold decrease in oxacillin MIC of MRSA strain USA300, rendering an MRSA to a MSSA phenotype. We propose to further characterize DNAC-23a with the following two specific aims: I) to identify the target of DNAC-23a; II) to conduct Structure-Activity-Relationship studies of DNAC-23a. The goal of these studies is to identify novel derivatives with improved efficacy, improved solubility and minimal toxicity. Future studies will focus on the optimal PK/PD values of these compounds, thus enabling us to identify a ?lead compound? for future drug development. As treatment options for resistant Gram+ pathogens are limited, discovery of novel compounds that extend the usage of current beta-lactams represents a potential advance in drug development.